1. The Field of the Invention
The present invention relates to self cleaning systems and methods for use in combustion devices. More particularly, the present invention relates to a self cleaning port system and method for reducing and removing ash deposits around a port in a combustion device such as in industrial and utility boilers or furnaces.
2. The Relevant Technology
In industrial and utility boiler or furnace combustion systems fired with fuels which contain ash such as coal, oil, biomass, and the like, ash deposits can form on the furnace walls. Depending on the characteristics of the ash and the thermal conditions in the furnace, the deposits can be solid or molten. Ash deposits can cause various problems in the operation of a combustion system, such as reducing heat transfer.
The conventional approach to control of furnace wall ash deposits involves the use of wall blowers which are short lances inserted through holes in the furnace wall. A material such as steam or air, which is termed the "blowing medium," is injected into the lance from outside the boiler or furnace and exits the lance through one or more holes in the lance which are directed parallel to or at an angle to the furnace wall. The blowing medium is injected at high velocity such that ash deposits on the furnace wall in the vicinity of the wall blower are dislodged from the surface of the furnace wall.
A number of wall blower designs have been developed in the past. The most common designs utilize a retractable lance which is normally recessed into the furnace wall. During a cleaning cycle, the lance is inserted into the furnace, the flow of the blowing medium is started and the lance is rotated so as to sweep a circular area of the furnace wall in the vicinity of the lance free from ash. At the end of the cleaning cycle, the lance is retracted back into the furnace wall.
Various types of equipment used in combustion systems require the injection of certain materials such as fuel, air, flue gas, nitrogen oxide (NO.sub.x) reduction agents, and sulfur dioxide (SO.sub.2) reduction agents into the furnace for the purpose of effecting combustion, pollutant emission control, and other functions. These materials are injected into the furnace through holes or ports in the furnace walls that can be configured in different ways. Examples of ports include burners where fuel and air are injected, overfire air ports where air is injected, flue gas recirculation ports where flue gas is injected, reburning ports where a reburn fuel is injected, pollutant emission control ports, and the like. In the discussion hereafter, the structure defining a hole or holes through the wall of a furnace where a material is injected is termed a "port" or "ports" and the material injected through the port is termed the "injected material." Such ports can include a single passageway or multiple passageways through a single hole in the furnace wall.
There are two basic types of furnaces used in combustion systems, including those with refractory covered walls and those with tubewalls. Most boilers have tubewalls where the furnace walls are formed of a series of vertical tubes side-by-side. The tubes can be directly welded together (tangent tubes), or there can be a short flat section welded between the tubes (membrane wall). The tubes can also be separate and stand in front of a refractory wall. In most applications the tubes are vertical, but in some applications the tubes run horizontally or at angles. In such tubewall furnaces, ports are formed by bending one or more tubes out of the plane of the tube wall and bending the adjacent tubes to form the port opening. The passageways for the injected material (e.g., fuel, air, NO.sub.x reduction agents, etc.) and burner components where applicable are inserted within the port opening.
In the normal operation of the ports in a combustion system, the flow of the injected material entrains furnace gases and suspended ash particles, thereby drawing such gases and particles to the entrance of the port where they can form ash deposits therearound on the furnace wall. These ash deposits are often detrimental to the operation of the port. A port with high velocity injection may exacerbate these local ash deposition problems. The size, shape and consistency of these deposits are often not symmetric and may be solid or molten. In some cases these deposits form "eyebrows" which may extend a considerable distance into the furnace, thereby affecting the operation of the port and changing furnace flow patterns. If these large deposits break off, they can fall to the furnace floor and cause structural damage. In other cases, molten ash deposits can drip over the port opening, partially or fully blocking the opening.
Conventional wall blowers are ineffective in removing the deposits around ports for several reasons. Such blowers are often located a considerable distance away from the ports due to mechanical interferences, reducing blower effectiveness. If the blowers are located close to the ports, the flow produced by the blowing medium can disrupt the normal operation of the port. The flow produced by conventional blowers tends to push deposits over the port opening, which can cause blockage, particularly with molten or partially molten deposits.
Thus, there is a need for a device which can be adapted to ports in a combustion system to provide for effective cleaning and ash deposit removal while minimizing any impact on the normal operation of the ports or other adverse effects.